This invention relates to optical transmission cables in general in more particularly to an improved optical communications cable which provides improved protection against tensile, compressive and flexural stresses.
In the recent years there has been a great deal of activity in the field of transmission of information using light waves. These developments have led to the possibility of using optical transmission cables utilizing glass fibers for guiding light beams. In other words such optical cables can be used in some instances to replace electrical transmission cables, the glass fibers being used instead of electrical conductors. Generally such glass fibers consist of a core made of a material with a hight index of refraction and a coat surrounding the core, the coat being made of a material having a smaller index of refraction. Optionally a protective outer jacket of polymer material such a varnish may be provided.
One type of optical cable utilizing a transmission element in which several glass fibers are arranged about a central plastic filament is known. The cross section of the plastic filament is developed to have several ribs with the glass fibers inserted along channels between adjacent ribs and secured in place by means of a plastic jacket. Such as disclosed in German Offenlegungsschrift No. 2,347,408 laid open Apr. 4, 1974 (Claim 1). Several such transmission elements can be used to construct an optical cable with the transmission elements arranged with an elongated central core of, for example, a multiplicity of twisted steel wires. In such a case the transmission elements are arranged about the steel core to form a layer. The purpose of the core is to provide reinforcement to take up tensile stresses. Over the layer or layers of the optical cable additional layers including insulated electrical conductors can be provided. Over the outside of the cable an outer plastic of an olefin ploymerisate or of polyethylene is placed. This jacket may then be surrounded with an additional jacket of a material of low coefficient of friction. Such an arrangement is disclosed in German Offenlegungsschrift No. 2,355,855 laid open May 16, 1974 (Claim 1, 11, 16, 18).
In another prior art optical cable the starting point is also a core built up of twisted steel wires for use as central reinforcing element. On this cable a layer of, for example, foamed plastic is first applied. Over this layer the optical transmission elements are arranged in one or more layers. For this purpose, the optical fibers are fastened into a flexible, flat mounting component which is wound in helical fashion around the central core. On top of the wound transmission elements is placed an additional layer of foamed plastic and following that an open braid of metal ribbon and the outside jacket of plastic, such as polyethylene. Such is disclosed in German Offenlegungsschrift No. 2,355,854 laid open May 16, 1974 (Claim 1, 17-21).
When designing optical cables, the sensitivity of the glass fibers used in the transmission elements to tensile, compressive and flexural stresses is of utmost importance. These factors are taken into account in the known cable design primarily by providing a core of high tensile strength and of cushioning intermediate layers between the core and transmission elements as well as providing cushioning between the elements and the outer jacket. Such however does not provide sufficient protection against tensile stresses and against radial compressive stresses exerted on the cable. Elongation of the core of high tensile strength steel when stressed must be withstood by the optical transmission elements. Complete protection against compression and tensile stresses, which in some circumstances may occur in the twisting of the transmission elements to form the cable core, is not adequately provided. Such complete protection which must also be sufficient for extended periods of operation under mechanical stress, is, however, a basic requirement for practical device. In view of this, the need for an improved optical cable, the transmission elements of which are protected to an extremely high degree against tensile, compressive and flexural stresses of all kinds becomes evident.